The present invention generally relates to organic thin-film photosensitive optoelectronic devices. More specifically, it is directed to organic photosensitive optoelectronic devices, e.g., solar cells and photodetectors, having transparent electrodes. Further, it is directed to organic photosensitive optoelectronic devices having a transparent cathode which forms a low resistance interface with the adjacent organic semiconductor layer. Further, it is directed to an organic photosensitive optoelectronic device consisting of a plurality of stacked photosensitive optoelectronic subcells, each subcell having one or more photosensitive optoelectronically active layers and transparent charge transfer layers.
Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as photovoltaic (PV) devices, are specifically used to generate electrical power. PV devices are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term xe2x80x9cresistive loadxe2x80x9d refers to any power consuming or storing device, equipment or system. Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light. Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector has a voltage applied and a current detecting circuit measures the current generated when the photodetector is exposed to electromagnetic radiation. A detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to ambient electromagnetic radiation. These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g. crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term xe2x80x9csemiconductorxe2x80x9d denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term xe2x80x9cphotoconductivexe2x80x9d generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms xe2x80x9cphotoconductorxe2x80x9d and xe2x80x9cphotoconductive materialxe2x80x9d are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation of selected spectral energies to generate electric charge carriers. Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs.
PV devices typically have the property that when they are connected across a load and are irradiated by light they produce a photogenerated voltage. When irradiated without any external electronic load, a PV device generates its maximum possible voltage, V open-circuit, or VOC. If a PV device is irradiated with its electrical contacts shorted, a maximum short-circuit current, or ISC, is produced. When actually used to generate power, a PV device is connected to a finite resistive load and the power output is given by the current voltage product, Ixc3x97V. The maximum total power generated by a PV device is inherently incapable of exceeding the product, ISCxc3x97VOC. When the load value is optimized for maximum power extraction, the current and voltage have values, Imax and Vmax. respectively. A figure of merit for solar cells is the fill factor ff defined as:                     ff        =                                            I              max                        ⁢                          V              max                                                          I              SC                        ⁢                          V              OC                                                          (        1        )            
where ff is always less than 1 since in actual use ISC and VOC are never obtained simultaneously. Nonetheless, as ff approaches 1, the device is more efficient.
When electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, for example, an organic molecular crystal (OMC) material, or a polymer, a photon can be absorbed to produce an excited molecular state. This is represented symbolically as S0+hvxe2x86x92S0*. Here S0 and S0* denote ground and excited molecular states, respectively. This energy absorption is associated with the promotion of an electron from a bound state in the valence band, which may be a xcfx80-bond, to the conduction band, which may be a xcfx80* -bond, or equivalently, the promotion of a hole from the conduction band to the valence band. In organic thin-film photoconductors, the generated molecular state is generally believed to be an exciton, i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. The excitons can have an appreciable life-time before geminate recombination, which refers to the process of the original electron and hole recombining with each other as opposed to recombination with holes or electrons from other pairs. To produce a photocurrent the electron-hole pair must become separated. If the charges do not separate, they can recombine in a geminate recombination process, either radiativelyxe2x80x94re-emitting light of a lower than incident light energyxe2x80x94, or non-radiativelyxe2x80x94with the production of heat.
Either of these outcomes is undesirable in a photosensitive optoelectronic device. While exciton ionization, or dissociation, is not completely understood, it is generally believed to occur in regions of electric field occurring at defects, impurities, contacts, interfaces or other inhomogeneities. Frequently, the ionization occurs in the electric field induced around a crystal defect, denoted, M. This reaction is denoted S0*+Mxe2x86x92exe2x88x92h+. If the ionization occurs at a random defect in a region of material without an overall electric field, the generated electron-hole pair will likely recombine. To achieve a useful photocurrent, the electron and hole must be collected separately at respective opposing electrodes, which are frequently referred to as contacts. This is achieved with the presence of an electric field in the region occupied by the carriers. In power generation devices, i.e., PV devices, this is preferably achieved with the use of internally produced electric fields that separate the generated photocarriers. In other photosensitive optoelectronic devices, the electric field may be generated by an external bias, e.g., in a photoconductor cell, or as a result of the superposition of internally and externally generated electric fields, e.g., in a photodetector. In an organic PV device, as in other photosensitive optoelectronic devices, it is desirable to separate as many of the photogenerated electron-hole pairs, or excitons, as possible. The built-in electric field serves to dissociate the excitons to produce a photocurrent.
FIG. 1 schematically depicts the photoconductive process in organic semiconducting materials. Step 101 shows electromagnetic radiation incident upon sample of photoconductive material between two electrodes a and b. In step 102, a photon is absorbed to generate an exciton, i.e., electron-hole pair, in the bulk. The solid circle schematically represents an electron while the open circle schematically represents a hole. The curving lines between the hole and electron are an artistic indication that the electron and hole are in an excitonic bound state. In step 103, the exciton diffuses within the bulk photoconductive material as indicated by the exciton""s closer proximity to electrode a. The exciton may suffer recombination in the bulk material away from any field associated with a contact or junction as indicated in step 104. If this occurs the absorbed photon does not contribute to the photocurrent. Preferably the exciton ionizes within the field associated with a contact or junction as indicated by the progression from step 103 to step 105. However, it is still possible for the newly liberated carriers to recombine as indicated in step 106 before permanently separating and contributing to the photocurrent. Preferably the carriers separate and respond to the field near a contact or junction according to the sign of their electric charge as indicated by the progression from step 105 to step 107. That is, in an electric field, indicated by xcex5 in step 107, holes and electrons move in opposite directions.
To produce internally generated electric fields which occupy a substantial volume, the usual method is to juxtapose two layers of material with appropriately selected conductive properties, especially with respect to their distribution of molecular quantum energy states. The interface of these two materials is called a photovoltaic heterojunction. In traditional semiconductor theory, materials for forming PV heterojunctions have been denoted as generally being of either n, or donor, type or p, or acceptor, type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states. The type of the background, i.e., not photogenerated, majority carrier concentration depends primarily on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the HOMO-LUMO gap. The Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to xc2xd. A Fermi energy near the LUMO energy indicates that electrons are the predominant carrier. A Fermi energy near the HOMO energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the prototypical PV heterojunction has traditionally been the p-n interface.
In addition to relative free-carrier concentrations, a significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. As opposed to free carrier concentrations, carrier mobility is determined in large part by intrinsic properties of the organic material such as crystal symmetry and periodicity. Appropriate symmetry and periodicity can produce higher quantum wavefunction overlap of HOMO levels producing higher hole mobility, or similarly, higher overlap of LUMO levels to produce higher electron mobility. Moreover, the donor or acceptor nature of an organic semiconductor, e.g., 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), may be at odds with the higher carrier mobility. For example, while chemistry arguments suggest a donor, or n-type, character for PTCDA, experiments indicate that hole mobilities exceed electron mobilities by several orders of magnitude so that the hole mobility is a critical factor. The result is that device configuration predictions from donor/acceptor criteria may not be borne out by actual device performance. Thus, in selecting organic materials such as those described herein for photosensitive optoelectronic devices, it has been found that isotype heterojunctions, e.g, p-p, may have rectifying properties as good as traditional p-n type heterojunctions, although true p-n type is generally preferable when possible. Isotype heterojunctions are discussed further below. Due to these unique electronic properties of organic materials, rather than designating them as xe2x80x9cp-typexe2x80x9d or xe2x80x9cn-typexe2x80x9d, the nomenclature of xe2x80x9chole-transporting-layerxe2x80x9d (HTL) or xe2x80x9celectron-transporting-layerxe2x80x9d (ETL) is frequently used. In this designation scheme, an ETL will preferentially be electron conducting and an HTL will preferentially be hole transporting. The term xe2x80x9crectifyingxe2x80x9d denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. Rectification is associated normally with a built-in electric field which occurs at the heterojunction between appropriately selected materials.
The electrodes, or contacts, used in a photosensitive optoelectronic device are an important consideration. In a photosensitive optoelectronic device, it is desirable to allow the maximum amount of ambient electromagnetic radiation from the device exterior to be admitted to the photoconductively active interior region. That is, it is desirable to get the electromagnetic radiation to where it can be converted to electricity by photoconductive absorption. This indicates that at least one of the electrical contacts should be minimally absorbing and minimally reflecting of the incident electromagnetic radiation. That is, the contact should be substantially transparent. When used herein, the terms xe2x80x9celectrodexe2x80x9d and xe2x80x9ccontactxe2x80x9d refer only to layers that provide a medium for delivering photogenerated power to an external circuit or providing a bias voltage to the device. That is, an electrode, or contact, provides the interface between the photoconductively active regions of an organic photosensitive optoelectronic device and a wire, lead, trace or other means for transporting the charge carriers to or from the external circuit. The term xe2x80x9ccharge transfer layerxe2x80x9d is used herein to refer to layers similar to but different from electrodes in that a charge transfer layer only delivers charge carriers from one subsection of an optoelectronic device to the adjacent subsection. As used herein, a layer of material or a sequence of several layers of different materials is said to be xe2x80x9ctransparentxe2x80x9d when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers.
When an electrode or charge transfer layer provides the primary mechanism for photovoltaic charge separation, the device is called a Schottky device as discussed further below.
Electrodes or contacts are usually metals or xe2x80x9cmetal substitutesxe2x80x9d. Herein the term xe2x80x9cmetalxe2x80x9d is used to embrace both materials composed of an elementally pure metal, e.g., Mg, and also metal alloys which are materials composed of two or more elementally pure metals, e.g., Mg and Ag together, denoted Mg:Ag. Here, the term xe2x80x9cmetal substitutexe2x80x9d refers to a material that is not a metal within the normal definition, but which has the metal-like properties that are desired in certain appropriate applications. Commonly used metal substitutes for electrodes and charge transfer layers would include wide bandgap semiconductors, for example, transparent conducting oxides such as indium tin oxide (ITO), tin oxide (TO), gallium indium tin oxide (GITO), zinc oxide (ZO) and zinc indium tin oxide (ZITO). In particular, ITO is a highly doped degenerate n+ semiconductor with an optical bandgap of approximately 3.2 eV rendering it transparent to wavelengths greater than approximately 3900 xc3x85. Another suitable metal substitute material is the transparent conductive polymer polyanaline (PANI) and its chemical relatives. Metal substitutes may be further selected from a wide range of non-metallic materials, wherein the term xe2x80x9cnon-metallicxe2x80x9d is meant to embrace a wide range of materials provided that the material is free of metal in its chemically uncombined form. When a metal is present in its chemically uncombined form, either alone or in combination with one or more other metals as an alloy, the metal may alternatively be referred to as being present in its metallic form or as being a xe2x80x9cfree metalxe2x80x9d. Thus, the metal substitute electrodes of the present invention may sometimes be referred to by one or more of the inventors of the present invention as xe2x80x9cmetal-freexe2x80x9d wherein the term xe2x80x9cmetal-freexe2x80x9d is expressly meant to embrace a material free of metal in its chemically uncombined form. Free metals typically have a form of metallic bonding that may be thought of as a type of chemical bonding that results from a sea of valence electrons which are free to move in an electronic conduction band throughout the metal lattice. While metal substitutes may contain metal constituents they are xe2x80x9cnon-metallicxe2x80x9d on several bases. They are not pure free-metals nor are they alloys of free-metals. Further, these metal substitutes do not have their Fermi level in a band of conducting states in contrast with true metals. When metals are present in their metallic form, the electronic conduction band tends to provide, among other metallic properties, a high electrical conductivity as well as a high reflectivity for optical radiation. Another characteristic of metallic conductors is the temperature dependence of their conductivity. Metals generally have a high conductivity at room temperature which increases as the temperature is lowered to near absolute zero. Metal substitutes, for example, semiconductors including, inter alia, inorganic, organic, amorphous, or crystalline, generally have conductivities which decrease as their temperature is lowered to near absolute zero.
There are two basic organic photovoltaic device configurations. The first type is the Schottky-type cell with a single species of organic photoconductive material sandwiched between a pair of metal and/or metal substitute contacts. Conventionally, for n-type photoconductors, a high work function metal, e.g., Au, has been used as the Schottky contact, and for p-type photoconductors, a metal with a low work function, e.g., Al, Mg, or In has been used as the Schottky contact. The charge separation desired in a PV device is induced by exciton dissociation in the space-charge region associated with the built-in electric field at the metal/photoconductor interface. Conventionally, such a device requires different metal or metal substitute pair combinations as contacts since use of the same material at both interfaces would ostensibly produce opposing rectifying junctions. If the same material is used for both electrodes it has been generally thought that the fields generated at the photoconductor-electrode interfaces are necessarily equal in magnitude and opposite in direction so that no net photocurrent is generated in the absence of an external applied voltage. While it is possible for charge separation to occur at both interfaces and be additive, it is generally preferable to have all charge separation occurring at one interface. For example, this can be achieved if the non-rectifying interface has little or no barrier to carrier transport, i.e., if it is a relatively low resistance contact. This may also be referred to as an xe2x80x9cohmicxe2x80x9d contact. In any event, in photosensitive optoelectronic devices it is generally desirable that the interfaces either contribute to the net charge separating action or present the smallest possible resistance or barrier to carrier transport.
A sample prior art organic Schottky device is shown schematically in FIG. 2A. Contact 2A01 is Ag; organic photoconductive layer 2A02 is PTCDA; and contact 2A03 is ITO. Such a cell was described in an article by N. Karl, A. Bauer, J Holzxc3xa4ofel, J Marktanner, M Mxc3x6bus, and F. Stxc3x6lzle, xe2x80x9cEfficient Organic Photovoltaic Cells: The Role of Excitonic Light Collection, Exciton Diffusion to Interfaces, Internal Fields for Charge Separation, and High Charge Carrier Mobilitiesxe2x80x9d, Molecular Crystals and Liquid Crystals, Vol. 252, pp 243-258, 1994 (hereinafter Karl et al.). Karl et al. noted that while the Ag electrode 2A01 was photovoltaically active in such a cell, the ITO electrode very rarely was photoactive and further that the indications of photovoltaic action at the ITO electrode had poor statistical certainty. Further, one of ordinary skill in the art would expect contact 2A01 not to be transparent.
The second type of photovoltaic device configuration is the organic bilayer cell. In the bilayer cell, charge separation predominantly occurs at the organic heterojunction. The built-in potential is determined by the HOMO-LUMO gap energy difference between the two materials contacting to form the heterojunction. An isotype heterojunction has been discussed in an article by S. R. Forrest, L. Y Leu, F. F. So, and W. Y. Yoon, xe2x80x9cOptical and Electrical Properties of Isotype Crystalline Molecular Organic Heterojunctionsxe2x80x9d Journal of Applied Physics, Vol. 66, No. 12, 1989 (hereinafter xe2x80x9cForrest, Leu et al.xe2x80x9d) and in an article by Forrest, S. R., xe2x80x9cUltrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniquesxe2x80x9d Chemical Reviews, Vol. 97, No. 6, 1997 (hereinafter Forrest, Chem. Rev. 1997) both of which are incorporated herein by reference. Forrest, Leu et al. describe two isotype solar cells depicted in FIG. 2B and FIG. 2C. FIG. 2B shows a device consisting of an ITO electrode 2B02 on a substrate 2B01 covered with a layer 2B03 of copper phthalocyanine (CuPc) and a layer 2B04 of PTCDA with a top electrode 2B05 of In. In a second device, with reference to FIG. 2C, an ITO electrode 2C02 is again placed on a substrate 2C01. Then a CuPc layer 2C03 and a 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) layer 2C04 are placed in order with a Ag electrode 2C05 on top. This prior art had only one transparent electrode and it was on the bottom of the device. It was also noted in this reference that these organic photovoltaic devices suffered from a high series resistance.
As in the case of Schottky devices, unless an interface, at a contact, for example, is contributing to the charge separation, it is desirable that the interface produce the smallest possible obstruction to free current flow. In bilayer devices, since the dominant charge separating region is located near the heterojunction, it is desirable for the interfaces at the electrodes to have the smallest possible resistance. In particular, it is known in the art to use thin metal layers as low resistance, or ohmic, electrodes, or contacts. When ohmic contacts are desired, a high work function metal, e.g., Au, has been used as the positive electrode, i.e., anode, in photosensitive optoelectronic devices. Similarly, a low work function metal, e.g., Al, Mg, or In, has been used to make an ohmic negative electrode, i.e., cathode.
Herein, the term xe2x80x9ccathodexe2x80x9d is used in the following manner. In a PV device under ambient irradiation and connected with a resistive load and with no externally applied voltage, e.g., a solar cell, electrons move to the cathode from the adjacent photoconducting material. With an applied bias voltage, electrons may move from the cathode to the adjacent photoconducting material, or vice versa, depending on the direction and magnitude of the applied voltage. For example, under xe2x80x9cforward-biasxe2x80x9d a negative bias is applied to the cathode. If the magnitude of the forward-bias equals that of the internally generated potential there will be no net current through the device. If the forward-bias potential exceeds the internal potential in magnitude there will be a current in the opposite direction from the non-biased situation. In this later forward-bias situation, electrons move from the cathode into the adjacent photoconductive organic layer. Under xe2x80x9creverse-biasxe2x80x9d, a positive bias is applied to the cathode and any electrons which can move do so in the same direction as in the no bias situation. A reverse-biased device generally has little or no current flow until it is irradiated. Similarly, the term xe2x80x9canodexe2x80x9d is used herein such that in a solar cell under illumination, holes move to the anode from the adjacent photoconducting material, which is equivalent to electrons moving in the opposite manner. The application of an external voltage to the device structure will alter the flow of the carriers at the anode/photoconductor interface in a complementary fashion to that described for the cathode and in a manner understood by those of ordinary skill in the art. It will be noted that as the terms are used herein anodes and cathodes may be electrodes or charge transfer layers.
Further, as discussed above, in non-Schottky photosensitive optoelectronic devices it is similarly desirable for the electrodes not merely to form ohmic contacts but also to have high optical transparency. Transparency requires both low reflectivity and low absorption. Metals have the desired low resistance contact properties; however, they can produce significant conversion efficiency reductions due to reflection of ambient radiation away from the device. Also, metal electrodes can absorb significant amounts of electromagnetic radiation, especially in thick layers. Therefore, it has been desirable to find low resistance, high transparency electrode materials and structures. In particular, the metal substitute ITO has the desired optical properties. It is also known in the art that ITO functions well as an anode in organic optoelectronic devices. However, it had not been previously thought that ITO or other metal substitutes could make low resistance cathodes for organic optoelectronic devices. Solar cells had been disclosed in which a highly transparent ITO layer may have functioned as a cathode in some cases, but such ITO cathodes were only disclosed as having been prepared by depositing the charge-carrying organic layer onto the ITO layer by Karl et al. and Whitlock, J. B., Panayotatos, P., Sharma, G.D., Cox, M. D., Savers, R. R., and Bird, G. R., xe2x80x9cInvestigations of Materials and Device Structures for Organic Semiconductor Solar Cellsxe2x80x9d, xe2x80x9cOptical Eng., Vol. 32, No. 8, 1921-1934 (August 1993), (Whitlock et al).
Prior art PV devices, e.g., FIG. 2A and 2B, have only utilized non-metallic materials, e.g., indium tin oxide (ITO), as one electrode of the photovoltaic device. The other electrode has traditionally been a non-transparent metallic layer, e.g., aluminum, indium, gold, tin, silver, magnesium, lithium, etc. or their alloys, selected on the basis of work function as discussed above. U.S. Pat. No. 5,703,436 to Forrest, S. R. et al. (hereinafter Forrest ""436), incorporated herein by reference, describes a technique for fabricating organic photoemissive devices (TOLEDs: Transparent Organic Light Emitting Diodes) having a transparent cathode deposited onto an organic ETL by depositing a thin metallic layer, e.g., Mg:Ag, onto the organic ETL and then sputter depositing an ITO layer onto the Mg:Ag layer. Such a cathode having the ITO layer sputter deposited onto a Mg:Ag layer is referred to herein as a xe2x80x9ccomposite ITO/Mg:Ag cathodexe2x80x9d. The composite ITO/Mg:Ag cathode has high transmission as well as low resistance properties.
It is known in the art of inorganic solar cells to stack multiple photovoltaic cells to create an inorganic multisection solar cell with transparent metallic layers. For example, U.S. Pat. No. 4,255,211 to Frass (hereinafter xe2x80x9cFrass ""211xe2x80x9d) discloses a stacked cell arrangement. However, the photolithographic techniques used to fabricate inorganic electronic devices are typically inapplicable to production of organic optoelectronic devices. Photolithography generally involves deposition of metallic layers and inorganic semiconductive layers followed by additional steps of masking and etching. The etching steps involve use of strong solvents which can dissolve the relatively fragile organic semiconductor materials that are suitable for organic photovoltaic devices. Therefore, organic photosensitive optoelectronic device fabrication techniques typically avoid this type of liquid etching process in which deposited material is removed after an organic layer has been deposited. Instead, device layers are generally deposited sequentially with techniques such as evaporation or sputtering. Access to electrodes is generally implemented using masking or dry etching during deposition. This constraint presents a challenge to fabrication of a stacked organic optoelectronic device for which electrode access to the intervening layers in the stack is desired. Thus, it is believed that all prior art stacked cells have the individual photovoltaic cells electrically connected internally and only in series.
For inorganic photovoltaic devices, series connection is not particularly disadvantageous. However, due to the high series resistance of the organic photovoltaic devices noted above, a series configuration is undesirable for power applications due to the reduced efficiency. Forrest, Chem. Rev. 1997 reported that high series resistance in organic solar cells leads to space-charge build-up as power levels are raised with increasing incident light intensity. This leads to degradation of the photocurrent, Imax effectively reducing the fill factor and therefore the efficiency. Moreover, what is believed to be the only previously disclosed organic solar cell with more than one photovoltaic subcell was a tandem, i.e., two PV subcells, with the subcells connected in series. See Effect of Thin Gold Interstitial-layer on the Photovoltaic Properties of Tandem Organic Solar Cell, Hiramoto, M; Suezaki, M; Yokoyama, M; Chemistry Letters 1990, 327 (hereinafter xe2x80x9cHiramotoxe2x80x9d). Referring to FIG. 2D, substrate 2D01 is glass; 2D02 is ITO; 2D03 is Me-PTC (500 xc3x85); 2D04 is H2Pc (700 xc3x85); 2D05 is Au( less than 30 xc3x85); 2D06 is Me-PTC (700 xc3x85); H2Pc (700 xc3x85); and 2D07 is Au (200 xc3x85). This device has the subcells electrically connected internally and in series, thus avoiding the problem of devising a means to make external contact to an electrode within the middle of a stack of organic semiconducting material. Hiramoto""s organic tandem devices have just two electrodes: one on top and bottom used to make external connections plus charge transfer layer 2D05 which electrically xe2x80x9cfloatsxe2x80x9d between the two subcells xe2x80x9d. Only one of the electrodes, bottom ITO layer 2D02 was transparent. Top Au layer 2D07 was 200 xc3x85 thick and therefore non-transparent. Further, for the reasons noted above, series connection is not an optimal configuration in stacked organic photovoltaic devices for high power applications.
A solar cell may be viewed as a photodiode with no applied bias. The internal electric field generates a photocurrent when light is incident on the solar cell and the current drives a resistive load for the extraction of power. On the other hand, a photodetector may be viewed as a diode with no externally applied bias voltage or a finite externally applied bias voltage. When electromagnetic radiation is incident upon a photodetector with a bias, the current increases from its dark value to a value proportional to the number of photogenerated carriers and the increase may be measured with external circuitry. If a photodiode is operated with no applied bias, an external circuit may be used to measure the photogenerated voltage and achieve photodetection. While the same general configuration of electrodes, charge transfer layers and photoconductive layers may be used alternatively as a solar cell or as a photodetector, a configuration optimized for one purpose is generally not optimal for another. For example, photosensitive optoelectronic devices produced as solar cells are designed to convert as much of the available solar spectrum as possible to electricity. Therefore, a broad spectral response over the entire visible spectrum is desirable. On the other hand, a photodetector may be desired which has a photosensitive response over a narrow spectral range or over a range outside the visible spectrum.
Organic PV devices typically have relatively low quantum yield (the ratio of photons absorbed to carrier pairs generated, or electromagnetic radiation to electricity conversion efficiency), being on the order of 1% or less. This is in part thought to be due to the second order nature of the intrinsic photoconductive process, that is, carrier generation requires exciton generation, diffusion and ionization, as described above. In order to increase these yields, materials and device configurations are desirable which can enhance the quantum yield and, therefore, the power conversion efficiency.
Forrest Chem. Rev. 1997 and Arbour, C., Armstrong, N. R., Brina, R., Collins, G., Danziger, J. -P., Lee, P., Nebesny, K. W., Pankow, J, Waite, S., xe2x80x9cSurface Chemistries and Photoelectrochemistries of Thin Film Molecular Semiconductor Materialsxe2x80x9d Molecular Crystals and Liquid Crystals, 1990, 183, 307, (hereinafter Arbour et al.), incorporated herein by reference in its entirety, disclose that alternating thin multilayer stacks of similar type photoconductors could be used to enhance photogenerated carrier collection efficiency over that using a single layer structure. Further, these sources describe multiple quantum well MQW) structures in which quantum size effects occur when the layer thicknesses become comparable to the exciton dimensions.
The present invention is directed to organic photosensitive optoelectronic devices utilizing transparent electrodes, in particular, devices that include an organic photosensitive optoelectronic cell comprised of at least one pair of two transparent electrodes, that is, a transparent cathode and a transparent anode, or devices that have a transparent electrode in superposed relationship upon the top surface of a substrate with at least one photoconductive organic layer disposed between the electrode and the substrate. More specifically, the organic photosensitive optoelectronic devices of the present invention may be comprised of a transparent cathode that is highly transparent and/or highly efficient. As representative embodiments, such transparent cathodes may be the highly transparent, highly efficient and/or low resistance non-metallic or metallic/non-metallic composite cathodes, such as disclosed in the co-pending application Ser. No. 08/964,863 hereinafter xe2x80x9cparthasarathy Appl. ""836xe2x80x9d and 09/054,707 hereinafter xe2x80x9cParthasarathy Appl. ""707xe2x80x9d or in Forrest ""436, each of which being incorporated in its entirety by reference.
The organic photosensitive optoelectronic devices of the present invention may function as a solar cell, photodetector or photocell. Whenever the organic photosensitive optoelectronic devices of the present invention function as solar cells, the materials used in the photoconductive organic layer or layers and the thicknesses thereof may be selected, for example, to optimize the external quantum efficiency of the device. Whenever the organic photosensitive optoelectronic devices of the present invention function as photodetectors or photocells, the materials used in the photoconductive organic layer or layers and the thicknesses thereof may be selected, for example, to maximize the sensitivity of the device to desired spectral regions. In each case, use of transparent electrodes, or even only a transparent top electrode, makes it possible for substantially higher external quantum efficiencies and/or photosensitivities in selected spectral regions to be realized compared to when one or more of the electrodes can cause substantial transmission losses due to absorption and/or reflection losses.
In addition to the organic photosensitive optoelectronic devices that may be comprised of two transparent electrodes or a transparent top electrode, the present invention is further directed to organic photosensitive optoelectronic devices having the unique geometric and electrical configurations that may be fabricated using stacked cells with transparent electrodes. In particular, the organic photosensitive optoelectronic device may be a stacked device comprised of a plurality of subcells in superposed relation to each other on the surface of a substrate. The materials and thicknesses of the individual subcells may be selected, for example, together with selecting the total number of subcells that are included in the stacked photosensitive optoelectronic device, so as to optimize the external quantum efficiency of the photosensitive optoelectronic device.
In particular, for stacked photosensitive optoelectronic devices configured to be electrically connected in parallel, the thicknesses of the individual subcells may be adjusted so that in combination with selecting the total number of subcells in the stacked device, the external quantum efficiency of the device may be optimized so as to obtain an external quantum efficiency that is higher than that which is possible for a single cell. The term xe2x80x9cexternal quantum efficiencyxe2x80x9d is used herein to refer to the efficiency with which a photosensitive optoelectronic device is capable of converting the total incident radiation into electrical power, as distinct from the term xe2x80x9cinternal quantum efficiency,xe2x80x9d which is used herein to refer to the efficiency with which a photosensitive optoelectronic device is capable of converting the absorbed radiation into electrical power. Using these terms, a stacked photosensitive optoelectronic device with an electrically parallel configuration may be designed to achieve an external quantum efficiency, under a given set of ambient radiation conditions, that approaches the maximum internal quantum efficiency that may be achieved for an individual subcell under such ambient conditions.
This result may be achieved by considering several guidelines that may be used in the selection of layer thicknesses. It is desirable for the exciton diffusion length, LD, to be greater than or comparable to the layer thickness, L, since it is believed that most exciton dissociation will occur at an interface. If LD is less than L, then many excitons may recombine before dissociation. It is further desirable for the total photoconductive layer thickness to be of the order of the electromagnetic radiation absorption length, 1/xcex1 (where xcex1 is the absorption coefficient), so that nearly all of the radiation incident on the solar cell is absorbed to produce excitons. However, the layer thicknesses should not be so large compared to the extent of the heterojunction electric fields that many excitons get generated in a field-free region. One reason for this is that the fields help to dissociate the excitons. Another reason is that if an exciton dissociates in a field-free region, it is more likely to suffer geminant recombination and contribute nothing to the photocurrent. Furthermore, the photoconductive layer thickness should be as thin as possible to avoid excess series resistance due to the high bulk resistivity of organic semiconductors.
Accordingly, these competing guidelines inherently require tradeoffs to be made in selecting the thickness of the photoconductive organic layers of a photosensitive optoelectronic cell. Thus, on the one hand, a thickness that is comparable or larger than the absorption length is desirable in order to absorb the maximum amount of incident radiation. On the other hand, as the photoconductive layer thickness increases, two undesirable effects are increased. One is that due to the high series resistance of organic semiconductors, an increased organic layer thickness increases device resistance and reduces efficiency. Another undesirable effect is that increasing the photoconductive layer thickness increases the likelihood that excitons will be generated far from the effective field at a charge-separating interface, resulting in enhanced probability of geminate recombination and, again, reduced efficiency. Therefore, a device configuration is desirable which balances between these competing effects in a manner that produces a high quantum efficiency for the overall device.
In particular, by taking the above-noted competing effects into account, that is, the absorption length of the photoconductive materials in the device, the diffusion length of the excitons in these materials, the photocurrent generation efficiency of these excitons, and the resistivity of these materials, the thickness of the layers in an individual cell may be adjusted so as to obtain a maximum internal quantum efficiency for those particular materials for a given set of ambient radiation conditions. Since the diffusion length of the excitons tends to have a relatively small value and the resistivity of typical photoconductive materials tends to be relatively large, an optimal subcell with respect to achieving the maximum internal quantum efficiency would typically be a relatively thin device. However, since the absorption length for such photoconductive organic materials tends to be relatively large as compared with the exciton diffusion length, such thin optimal photosensitive optoelectronic subcells, which may have the maximum internal quantum efficiency, would tend to have a relatively low external quantum efficiency, since only a small fraction of the incident radiation would be absorbed by such optimal subcells.
So as to improve the external quantum efficiency of an individual subcell, the thickness of the photoconductive organic layers may be increased so as to absorb significantly more incident radiation. Although the internal quantum efficiency for converting the additionally absorbed radiation into electrical power might gradually decrease as the thickness is increased beyond its optimal subcell thickness, the external quantum efficiency of the subcell would still increase until a certain thickness is reached where no further increase in absorption could produce an increase in external quantum efficiency. Since the internal quantum efficiency of the subcell tends to drop rather sharply as the thickness of the photoconductive layers increases much beyond the diffusion length of the photogenerated excitons, the maximum external quantum efficiency of the subcell may be achieved well before the thickness of the thicker subcell is sufficient to absorb substantially all the incident radiation. Thus, the maximum external quantum efficiency that may be achieved using this single, thicker-cell approach is limited not only by the fact that the subcell thickness may be significantly greater than that desired for achieving the maximum internal quantum efficiency but, in addition, such thicker subcells may still not absorb all the incident radiation. Thus, due to both of these effects, the maximum external quantum efficiency of the thicker subcell would be expected to be significantly less than the maximum internal quantum efficiency that can be achieved for an optimal subcell having the optimal thickness.
A particular feature of the present invention having the stacked organic photosensitive optoelectronic device with the electrically parallel configuration is that instead of attempting to improve the external quantum efficiency by increasing the thickness of a single subcell, which sacrifices the internal quantum efficiency, subcells that have a thickness that is optimal or near optimal for achieving the maximum internal quantum efficiency may be used to fabricate a stacked structure. The total number of such optimal subcells that are included in the stacked structure may be increased so as to provide an increase in absorption of the incident radiation with the total number being limited by that which produces no further increase in the external quantum efficiency. The net result of this approach for improving the external quantum efficiency is that a stacked organic photosensitive optoelectronic device can be made to have an external quantum efficiency approaching the maximum value of the internal quantum efficiency that can be achieved for an individual optimal subcell. The improved external quantum efficiency of the stacked devices may be attributed in large part to the fact that the subcells of the stacked device may be comprised of pairs of transparent electrodes and, in some cases, also of a transparent top electrode.
Taking into account that the additional subcells of the stacked device tend to introduce additional losses, such as that due to the residual reflectivity of the transparent electrodes, the maximum external quantum efficiency that can be achieved for a fully optimized stacked device would typically be somewhat less than the internal quantum efficiency of an optimal subcell. Nevertheless, using the methods of the present invention for optimizing the external quantum efficiency of an organic photosensitive optoelectronic device, substantially higher external quantum efficiencies may be achieved for a stacked device than are possible for a device having a single cell, which is optimized for external quantum efficiency at the expense of internal quantum efficiency.
Since the organic photosensitive optoelectronic devices of the present invention may be desired for widely varying ambient radiation conditions, for example, with respect to the intensity of incident radiation and/or with respect to the spectral distribution of the incident radiation, the photoconductive organic materials, and the layer thicknesses thereof, may be selected so as to be optimized for a given set of ambient conditions. For example, the photoconductive organic materials may be selected to have absorption maxima in selected spectral regions. Since the photoconductive organic materials that may be used in a photosensitive optoelectronic cell may typically have absorption maxima only over a limited spectral range, it is an additional feature of the present invention that the stacked photosensitive optoelectronic devices may be comprised of different types of cells having photoconductive organic materials with different absorption characteristics so as to more effectively utilize the entire spectral range of the incident radiation.
When the term xe2x80x9csubcellxe2x80x9d is used hereafter, it may refer to a organic photosensitive optoelectronic construction of the unilayer, bilayer or multilayer type. When a subcell is used individually as a photosensitive optoelectronic device, it typically includes a complete set of electrodes, i.e., positive and negative. As disclosed herein, in some stacked configurations it is possible for adjacent subcells to utilize common, i.e., shared, electrode or charge transfer layers. In other cases, adjacent subcells do not share common electrodes or charge transfer layers. The term xe2x80x9csubcellxe2x80x9d is disclosed herein to encompass the subunit construction regardless of whether each subunit has its own distinct electrodes or shares electrodes or charge transfer layers with adjacent subunits. Herein the terms xe2x80x9ccellxe2x80x9d, xe2x80x9csubcellxe2x80x9d, xe2x80x9cunitxe2x80x9d, xe2x80x9csubunitxe2x80x9d, xe2x80x9csectionxe2x80x9d, and xe2x80x9csubsectionxe2x80x9d are used interchangeably to refer to photoconductive layer or set of layers and the adjoining electrodes or charge transfer layers. As used herein, the terms xe2x80x9cstackxe2x80x9d, xe2x80x9cstackedxe2x80x9d, xe2x80x9cmultisectionxe2x80x9d and xe2x80x9cmulticellxe2x80x9d refer to any optoelectronic device with multiple layers of a photoconductive material separated by one or more electrode or charge transfer layers.
Since the stacked subcells of the solar cell may be fabricated using vacuum deposition techniques that allow external electrical connections to be made to the electrodes separating the subcells, each of the subcells in the device may be electrically connected either in parallel or in series, depending on whether the power and/or voltage generated by the solar cell is to be maximized. The improved external quantum efficiency that may be achieved for the stacked solar cells of the present invention may also be attributed to the fact that the subcells of the stacked solar cell may be electrically connected in parallel since a parallel electrical configuration permits substantially higher fill factors to be realized than when the subcells are connected in series. It is believed that this parallel electrical configuration of the stacked subcells is a further unique aspect of the present invention.
Although the high series resistance of photoconductive organic materials inhibits use of subcells in a series configuration for high power applications, there are certain applications, for example, in operating liquid crystal displays (LCD), for which a higher voltage may be required, but only at low current and, thus, at low power levels. For this type of application, stacked, series-connected solar cells may be suitable for providing the required voltage to the LCD. In the case when the solar cell is comprised of subcells electrically connected in series so as to produce such a higher voltage device, the stacked solar cell may be fabricated so as to have each subcell producing approximately the same current so to reduce inefficiency. For example, if the incident radiation passes through in only one direction, the stacked subcells may have an increasing thickness with the outermost subcell, which is most directly exposed to the incident radiation, being the thinnest. Alternatively, if the subcells are superposed on a reflective surface, the thicknesses of the individual subcells may be adjusted to account for the total combined radiation admitted to each subcell from the original and reflected directions.
Further, it may be desirable to have a direct current power supply capable of producing a number of different voltages. For this application, external connections to intervening electrodes could have great utility and are not believed to have been previously disclosed. Accordingly, in addition to being capable of providing the maximum voltage that is generated across the entire set of subcells, the stacked solar cells of the present invention may also be used to provide multiple voltages from a single power source by tapping a selected voltage from a selected subset of subcells.
The present invention may be further described as being directed toward a method of fabricating photosensitive optoelectronic devices comprising fabricating a first photosensitive optoelectronic subcell on a substrate so as to form a photosensitive optoelectronic cell capable of producing a given external quantum efficiency, and fabricating a second photosensitive optoelectronic subcell in superposed relationship upon the top surface of the first photosensitive optoelectronic subcell so as to form a stacked photosensitive optoelectronic device so as to increase the external quantum efficiency capability of the photosensitive optoelectronic cell, wherein at least one of the subcells of the stacked photosensitive optoelectronic cell is comprised of a pair of transparent electrodes.
The present invention may be further described as being directed toward a method of fabricating a series stacked organic photosensitive optoelectronic device comprising fabricating a first organic photosensitive optoelectronic subcell on a substrate so as to form an organic photosensitive optoelectronic device capable of producing a given voltage, and fabricating a second organic photosensitive optoelectronic subcell in superposed relationship upon the top surface of the first organic photosensitive optoelectronic subcell so as to form a stacked organic photosensitive optoelectronic device and so as to increase the voltage capability of the organic photosensitive optoelectronic device, wherein the subcells of the stacked organic photosensitive optoelectronic cell are comprised of a pair of transparent electrode layers and the first subcell and the second subcell are electrically connected in series.
The present invention may be further described as being directed toward a method of fabricating a parallel stacked organic photosensitive optoelectronic device comprising fabricating a first organic photosensitive optoelectronic subcell on a substrate so as to form an organic photosensitive optoelectronic device capable of producing a given external quantum efficiency, and fabricating a second organic photosensitive optoelectronic subcell in superposed relationship upon the top surface of the first organic photosensitive optoelectronic subcell so as to form a stacked organic photosensitive optoelectronic device so that the external quantum efficiency capability of the organic photosensitive optoelectronic device is increased, wherein the first subcell and the second subcell are electrically connected in parallel.
The present invention may be further described as being directed toward a mixed electrical configuration stacked organic photosensitive optoelectronic device comprising a substrate having a proximal surface and a distal surface, and a plurality of subassemblies of organic photosensitive optoelectronic subcells, each of the subcells having a cathode and an anode, each of the cathode and anode being an electrode layer or a charge transfer layer, the subcells in superposed relation with each other and with the distal surface of the substrate, each of the subassemblies of subcells comprising a plurality of subcells electrically connected in parallel or a plurality of subcells electrically connected in series, wherein the subassemblies are electrically connected to each other in series or in parallel such that the device includes subcells electrically arranged in series and parallel, so that the device is capable of producing a voltage higher than possible with a completely parallel arrangement with the same materials and with higher external quantum efficiency than a completely series arrangement for producing the same voltage.
The present invention may be further described as being directed toward a method of fabricating a mixed electrical configuration stacked organic photosensitive optoelectronic device comprising: fabricating a first organic photosensitive optoelectronic subcell on a substrate so as to form an organic photosensitive optoelectronic device; fabricating a second organic photosensitive optoelectronic subcell in superposed relationship upon the top surface of the first organic photosensitive optoelectronic subcell so as to form a first stacked organic photosensitive optoelectronic subassembly comprised of the first subcell and the second subcell electrically connected in series; fabricating a third organic photosensitive optoelectronic subcell in superposed relationship upon the top surface of the second organic photosensitive optoelectronic subcell; and fabricating a fourth organic photosensitive optoelectronic subcell in superposed relationship upon the top surface of the third organic photosensitive optoelectronic subcell so as to form a second stacked organic photosensitive optoelectronic subassembly comprising the third subcell and the fourth subcell electrically connected in series, wherein the first stacked organic photosensitive optoelectronic subassembly and the second stacked organic photosensitive optoelectronic subassembly are electrically connected in parallel.
Representative embodiments may also comprise transparent charge transfer layers. As described herein charge transfer layers are distinguished from ETL and HTL layers by the fact that charge transfer layers are frequently, but not necessarily, inorganic and they are generally chosen not to be photoconductively active. That is, the electrodes and charge transfer layers preferably do not absorb electromagnetic radiation for conversion to electrical or thermal forms of energy. Therefore, transparent low reflectivity electrodes and charge transfer layers are generally preferred in the present invention. In addition, the electrode and charge transfer layer electronic properties are important. In certain device configurations one or more of the electrodes or charge transfer layers may be electronically active. For example, as discussed above, an electrode or charge transfer layer may provide an interfacial region for dissociating or recombining excitons, or it may provide a rectifying interface. In other device configurations, it is desired that the electrode or charge transfer layer have minimal electronic activity and instead serve primarily as a low resistance means for delivering the photogenerated current to the external circuitry or to the adjacent subsection of a multisection device. Moreover, in PV devices, high contact or charge transfer layer resistance is detrimental in many applications since the resulting increased series resistance limits power output.
The preferred embodiments of the present invention include, as one or more of the transparent electrodes of the optoelectronic device, a highly transparent, non-metallic, low resistance cathode such as disclosed in Parthasarathy Appl. ""707 or a highly efficient, low resistance metallic/non-metallic composite cathode such as disclosed in Forrest ""436. Each type of cathode is preferably prepared in a fabrication process that includes the step of sputter depositing an ITO layer onto either an organic material, such as copper phthalocyanine (CuPc), PTCDA and PTCBI, to form a highly transparent, non-metallic, low resistance cathode or onto a thin Mg:Ag layer to form a highly efficient, low resistance metallic/non-metallic composite cathode. Parthasarathy Appl. ""707 discloses that an ITO layer onto which an organic layer had been deposited, instead of an organic layer onto which the ITO layer had been deposited, does not function as an efficient cathode.
In summary, it is an object of the present invention to provide an organic photosensitive optoelectronic device with two transparent electrodes.
More specifically, it is an object of the present invention to provide a stacked solar cell comprised of one or more subcells comprised of two transparent electrodes.
It is another object of the present invention to provide a stacked solar cell capable of operating with a high external quantum efficiency.
It is a still more specific object of the present invention to provide a stacked solar cell capable of operating with an external quantum efficiency that approaches the maximum internal quantum efficiency of an optimal PV subcell.
It is yet another object of the present invention to provide a stacked solar cell capable of operating with a higher voltage than can be provided by a single subcell.
Another object of the present invention is to provide an organic photosensitive optoelectronic device including multiple quantum well structures.
A further object of the present invention is to provide a stacked organic photosensitive optoelectronic device comprised of multiple organic photosensitive optoelectronic subcells with the subcells having external electrical connections.
Another object of the present invention is to provide an organic photosensitive optoelectronic device with improved absorption of incident radiation for more efficient photogeneration of charge carriers.
It is a further objective of the present invention to provide an organic photosensitive optoelectronic device with an improved VOC and an improved ISC.
Another object of the present invention is to provide a stacked organic photosensitive optoelectronic device having parallel electrical interconnection of the subcells.
A further object of the present invention is to provide a stacked organic photosensitive optoelectronic device comprised of multiple organic photovoltaic subcells with transparent electrodes and having a substantially reflective bottom layer to increase overall electromagnetic radiation absorption by capturing the electromagnetic radiation reflected by the bottom layer.
Yet another object of the present invention is to provide organic photosensitive optoelectronic devices including a conductive or an insulating substrate.
A further object of the present invention is to provide organic photosensitive optoelectronic devices including a rigid or a flexible substrate.
A further object of the present invention is to provide organic photosensitive optoelectronic wherein the organic materials used are polymeric or non-polymeric thin films.